Children with attention-deficit hyperactivity disorder (ADHD) exhibit substantial impairment in key domains that are of developmental significance, such as academic achievement, family interaction, and peer relationships. Nearly the entire literature on ADHD, however, has focused on males; the data base on impairment, associated features, comorbidity, mediators, and underlying mechanisms in females is quite small. Furthermore, almost all of the pertinent literature has focused on establishing male-female differences in mean levels of behavioral, psychological, or treatment-related variables; far less is known about those processes that mediate psychopathology, impairment, and social competence in girls. In addition, little is known about subtype differences (i.e., Inattentive vs. Combined types) in ADHD girls with regard to manifestations, impairment, and mechanisms of psychopathology. Based on the PI's published studies on boys with ADHD, a naturalistic and rigorous methodology is proposed for investigations of girls with ADHD, particularly related to comorbid antisocial behavior, internalizing features, parenting practices and attitudes, sociocognitive variables, neuropsychological performance, and peer status. Combined-type and inattentive-type females, aged 7-12 years, will be directly compared with one another and with non-ADHD comparison girls of the same age range. Through validated assessment procedures, including documentation of comorbidity, and participation in ecologically-valid research summer programs containing equal numbers of ADHD and comparison females, mechanisms of psychopathology and mediators of key areas of impairment and competence will be examined via parametric analyses of subgroup differences, hierarchical multiple regression analyses, and structural equation models. Through objective observations of social behavior, reliable individual interviews of internalizing symptomatology and sociocognitive domains, observed and self-reported parenting styles, and peer appraisal of social status (affording non-shared method variance across key areas of interest), the overall goal is to uncover mediators of competence and impairment in girls with different forms of ADHD, as well as in comparison girls, towards the end of elucidating underlying mechanisms of normal and atypical development.